Compact multibeam laser light source and interleaving raster scan line method for exposing printing plates

ABSTRACT

An individually drivable array of single stripe laser diodes is proposed for imaging printing plates. An imaging optics is used to produce n image points which have a spatial interval l between adjacent points. An interleaving raster scan line method is indicated, which, given proper selection of the increment, enables each dot to be exposed exactly once.

BACKGROUND INFORMATION

[0001] The present invention is directed to a device for imagingprinting plates using an array of n laser diodes.

[0002] For some time now, devices and methods have been known, whichmake it possible to image a printing plate, whether it be a flat orcurved surface, through exposure to laser radiation. Devices and methodsof this kind are used, in particular, in so-called CtP systems,computer-to-plate, or direct imaging print units or printing presses formanufacturing offset printing forms.

[0003] At the present time, printing plates are primarily imaged bylaser diode systems. Their inherent system properties prevent them fromreaching the physical limits of the beam quality. In particular, theirlow beam quality limits their depth of focus, so that an autofocusingsystem is needed at high resolutions. Two different approaches arecurrently used for multibeam imaging, i.e., for simultaneously exposinga plurality of image points on various media, such as printing plates,films, data carriers or the like. On the one hand, the radiation fromindividual laser diodes or an array of laser diodes can be directlyapplied via optical elements, such as lenses, mirrors or fibers, to themedium to be imaged. On the other hand, the radiation from a laser lightsource, typically laser diode bars, can be projected via diverse opticalelements onto an array of n modulators. For the most part, these areelectrooptical or acoustooptic modulators. By selectively driving the nmodulators, one can select individual beams from the entire radiationand modulate their power. The selected, power-modulated beams aresupplied via further optical elements to the medium to be imaged.

[0004] European Patent Application No. 0 878 773 A2 describes an opticalsystem for imaging an array of light sources, in particular anindividually addressable array of laser substantially greater than theiremitter height. The emission region is typically about 1 micrometer highand 60 micrometers wide. The optical system is composed of a system ofnon-anamorphotic imaging lenses and of a cylinder lens, which is placedbetween the array and the imaging lens system and images the laserradiation onto the scanning surface. This surface usually does not liein the foci of the laser beams, so that a widening of the shortdimensions of the imaged emission surface occurs.

[0005] U.S. Pat. No. 5,521,748 describes a system for exposing imagedata using an individual laser or an array of diodes and a lightmodulator. The light transmitted by the laser or the array is imagedonto a modulator having a row of light-modulating elements of thereflectance or transmittance type. Once selection and power modulationare carried out, the radiation is imaged onto a surface havinglight-sensitive material, forming individual image points. To placeimage points of this kind on a complete, two-dimensional surface, arelative motion of the image points to the light-sensitive material isprovided. In the interplay resulting from generation of the individualpoints and the relative motion, the desired image data are then exposedon the two-dimensional surface. The relative motion between the lightbeams emanating from the light modulator and the light-sensitivematerial can be effected on a cylindrical configuration such that linesare exposed in a meander shape along the axis of symmetry of thecylinder, or such that lines run around the cylinder in a helical form.

[0006] U.S. Pat. No. 5,691,759 discusses a multi-beam laser lightsource, which produces raster scan lines on a medium using the so-calledinterleaving raster scan line method. The interleaving raster scan linemethod is distinguished by the following properties. A laser lightsource emits radiation, from which n image points are produced usingmodulated power by employing suitable imaging optics and modulation.These n image points are arranged in a row, and the distance between twoadjacent points is (n+1)p, p being the distance between the dots.Provision is made between the medium and the image points for a relativemotion in both directions, spanning the surface of the medium. Once npoints are imaged, the medium is displaced relatively to the imagepoints with a translational component that is perpendicular to thedirection defined by the axis of the image points, so that n points canagain be exposed at another location of the medium. In this manner,so-called scan lines of image points are formed, initially at a distanceof (n+1)p, which are produced by laser radiation, whose power ismodulated in dependence upon the image information. Upon completion of ascan having a translational component in the perpendicular direction, adisplacement by the distance (n×p) follows in parallel to the directiondefined by the axis of the n image points. The n image points are thenshifted again with a translational component that is perpendicular tothe direction defined by the axis of the image pixels on the surface,forming further scan lines. Thus, each raster scan line is separatedfrom its immediate neighbor by the pitch distance p between the dots.Using a plurality of optical beams from a laser light source, anoverlapping of the scan lines ensues (interleaving raster scan linemethod).

[0007] An enhanced interleaving raster scan line method for a multibeamlaser light source is described in European Patent Application No. 0 947950 A2. In the case of n image points having a pitch distance p of thedots, each of whose adjacent image points are separated by the distance(q×n+1)p, q being a natural number, an incremental distance of n×presults by which the medium must be moved between the marking of twoscan lines. An overlapping (interleaving) of the scan lines is therebyachieved, in other words, the new scan lines are written between the oldscan lines. By properly selecting the displacement in parallel to theaxis defined by the image points, by the distance n×p, an imaging isthen possible, without one location, where image information is to bescanned, being repeatedly exposed to one image point of a laser. Whatdistinguishes the described method is that adjacent image points of thelaser diodes are spaced further apart, in each case, than the width ofthe displacement by which the medium is moved between the old and newscan lines.

[0008] Various disadvantages are associated with each of the knowndevices. The radiation emitted by broad array laser diodes, laser diodebars, and laser diode stacks exhibits a low beam quality, as quantifiedby the diffraction index M². Even with correction, the attainable depthof focus is only suited for imaging at a low resolution, typically 1,270dpi. Therefore, to produce very small dots, for example resolutions ofabout 2,540 dpi, an autofocusing system is necessary, which requires acomplex mechanical and electrical design. If the light source andmodulator are provided separately, there is an increased requirement foroptical, electronic and mechanical components, as well as forsubstantial overall space. Many components need to be adjusted, and theservice life can be clearly limited. The temperature management of thecomponents turns out to be just as problematic. Only a limited, minimalphysical size is possible when a device for imaging printing plates isassembled from discrete components. The described interleaving rasterscan line method is not suited for compact laser light sources, sincethe distance between adjacent image points must always be one unit pgreater than the number of beams, so that one must revert to scanningmethods in which image points are set densely together.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a device forimaging printing plates using an array of n laser diodes, whose emittedlight exhibits a good beam quality and which renders possible a compactdesign. An additional or alternate object of the present invention is toprovide an improved interleaving raster scan line method.

[0010] The present invention provides a device for imaging printingplates using an array (10) of n laser diodes which are imaged onto nimage points (110), so that one laser diode (12) is allocated to eachi-th point having i from {1, . . . , n}, the n image points (110) beingseparated by a spatial interval of adjacent points l, and a pitchdistance p of the dots being provided. The laser diodes (12) areindividually drivable single stripe laser diodes.

[0011] The present invention also provides a method, i.e., a so-calledinterleaving raster scan line method, for imaging printing plates bygenerating raster points using an array of n laser light sources, whichare imaged using an imaging optics onto n image points, which arearranged on a line, the n image points being separated by a spatialinterval of adjacent points l, comprising the following method steps:

[0012] simultaneous generation of n image points on the printing plateby a number of laser light sources;

[0013] generation of a relative motion between the image points andprinting plate;

[0014] displacement of the image points with a translation componentperpendicular to the axis defined by the line of the image points by afirst specific amount;

[0015] displacement of the n image points in the direction defined bythe n image points by a second specific amount; and

[0016] iteration of the displacements in question, wherein the amount ofthe second specific displacement is greater than the spatial interval lof adjacent image points.

[0017] In accordance with the present invention, the device for imagingprinting plates includes an array of n single stripe laser diodes. Eachsingle stripe laser diode can be driven individually. The n laser beamscan preferably be imaged onto the medium using light-transmitting means,such as lenses, mirrors, optical fibers or the like. The n image pointsproduced with the assistance of imaging optics are advantageouslydisposed on a line and have a spatial interval l between adjacentpoints. It is generally only necessary, however, that the n image pointsprojected onto a predefined line in the surface of the printing platehave a constant spatial interval l. A relative motion takes placebetween the medium and the image points in both directions, spanned bythe surface of the medium. In addition to the motion, which, in order todisplace the image points with a translational component perpendicularto the direction defined by the line of the n image points or by thepredefined line, on which the projected n image points exhibit aconstant spatial interval l, a displacement takes place in parallel tothe direction defined by line of the n image points or by the predefinedline, on which the projected n image points exhibit a constant spatialinterval l. The amount of this displacement is advantageously greaterthan or equal to the spatial interval l between the n image points.Raster scan lines are produced which exhibit a pitch distance p betweenthe dots, pitch distance p between the dots being smaller than spatialinterval l between the image points.

[0018] One preferred specific embodiment provides that the power supplyto the array of the laser diodes be regulated. A suitable detectorelement advantageously checks for proper functioning, and, as the casemay be, for potential malfunctioning of a single stripe laser diode,either on the outcoupling side of the laser diode or, however, atanother cavity mirror. In this context, the detector element can be botha detector row, as well as an individual detector, which scans theindividual single stripe laser diodes.

[0019] One derives a number of advantages from the use of an array of nsingle stripe laser diodes, which can be individually driven, and fromthe application of the corresponding interleaving raster scan linemethod to image printing plates. An excellent beam quality is achievedby using single stripe laser diodes. Typically, the value of diffractionindex M² is slightly higher than one. In a compact design, a high levelof integration can be achieved: radiation source, modulation, andcontrol can be combined in one component. The result is fewer opticalcomponents and, therefore, less need for adjustment of sensitivecomponents. The service life of the component is essentially limitedonly by the service life of the laser. The compact, modular design makesthe system scalable. A high-performance stability is assured by a rapidcontrol. The high level of integration provides for a simplertemperature management, since it is only necessary to cool this onecomponent. Due to the low diffraction index M², a maximum possible depthof focus is achieved when focusing.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Further advantages and beneficial specific embodiments of thepresent invention are presented on the basis of the following figures,as well as their descriptions, in which:

[0021]FIG. 1 shows a schematic view of the typical geometry in theimaging of a printing plate by an array of laser diodes having n laserbeams;

[0022]FIG. 2 shows a schematic view of the imaging of a printing plateon a cylinder by an array of n laser beams; and

[0023]FIG. 3 shows an example of imaging including an array of fiveimage points in the interleaving raster scan line method.

DETAILED DESCRIPTION

[0024]FIG. 1 depicts a typical geometry for projecting n laser lightbeams which emanate from an array of n laser diodes. Light source 10 iscomposed of an individually drivable array of n single stripe laserdiodes 12. It is customary for a light source of this kind to have up to100 single stripe laser diodes, advantageously between 10 and 60. Thesingle stripe laser diodes have emitter surfaces of a typical size of1×5 micrometers², and emitted laser radiation of an advantageous beamquality with a low diffraction index M². The individual laser diodes areusually spaced apart on the array by distances of between 100 and 1000micrometers. Imaging optics 16 is used to project the n laser beams onton image points 110 on a plate 18. Printing plate 18 is advantageouslysituated in the foci of laser beams 14. It is of particular benefit thatimaging optics 16 not only modifies the diameter proportions of thelaser beams (perpendicularly and in parallel to the axis defined by then points), but that it also corrects the distance by which the imagepoints are set apart from one another. In other words, both the spotsize of n image points 110, as well as their position relative to oneanother and their spatial interval are adjustable. As a general rule,the individual laser diodes are spaced apart by a constant distance;however, the minimum requirement for an advantageous imaging is onlythat spatial interval l of n image points 110 be constant. The spatialinterval l among the n image points is greater than the pitch distance pamong the dots.

[0025] Light source 10 can be used in continuous operation. To produceindividual light packets, the laser emission is suppressed accordinglyby a specific time interval. One specific embodiment also provides,however, for employing a light source 10 which emits pulsed radiation.When working with pulsed radiation, the repetition rate of the lightpulses must be at least exactly as great as the pulse frequency used togenerate the individual dots, so that at least one laser pulse isavailable for one dot. Imaging optics 16 can have reflective,transmittive, refractive, or similar optical components. These arepreferably micro-optical components. Imaging optics 16 can be bothenlarging as well as reducing, and also have different imaging scales inboth directions, in parallel and perpendicular to the active zone of thelasers. This is particularly beneficial for correcting divergence andaberration. The physical or chemical properties of the surface ofprinting plate 18 are modified by the laser radiation. Printing plateswhich are erasable or rewritable are advantageously used.

[0026] In one preferred specific embodiment, light source 10 is disposedon a cooling element 112. Light source 10 is linked via a current-supplyand control line 114 to control unit 116. Control unit 116 hasindividual components, which enable individual laser diodes of the arrayto be driven or regulated separately from one another. Cooling element112 is linked via a line for controlling cooling element 118 totemperature control 120.

[0027] A detector 122 is provided to test for correct functioning anddetermine the power output of individual laser diodes 12. The design ofthe detector can be such that an individual measuring device is providedfor each laser diode or, however, that a measuring device checks theindividual laser diodes at the time of replacement or as needed.Detector 122 is advantageously linked via connection 124 to control unit116, so that the power output is processed, inter alia, as a parameterfor generating a control signal in laser control 116.

[0028] A device of this kind in accordance with the present inventioncan be provided as an internal device in a print unit or a printingpress, or be provided externally thereto.

[0029]FIG. 2 illustrates the imaging of a printing plate, which issituated on a rotatable cylinder. Light source 20 produces n laser beams22, which are projected using imaging optics 24 onto n image points 210.The n image points are uniformly spaced apart and are disposed on oneaxis. Printing plate 28 is situated on a cylinder 26, which is rotatableabout its axis of symmetry 25. This rotation is denoted by arrow B.Light source 20 can be moved in parallel to axis of symmetry 25 of thecylinder on a linear path shown by double arrow A. For continuousimaging, cylinder 26 rotates with printing plate 28 in accordance withrotational motion B, and the translation of the light source along thecylinder is in accordance with moving direction A. The feed rate isdetermined by the number of laser beams 22 and the width p of a dot. Theresult is an imaging which encircles axis of symmetry 25 of cylinder 26on a helical path. The path of image points 210 is indicated by lines212. In other words, once imaging of n points is complete, a relativedisplacement of printing plate 28 and image points 210 follows with avector component perpendicular to the direction defined by the line ofthe n image points, by a first specific amount, so that n points areexposed again at another position of printing plate 28. In this manner,so-called raster scan lines of image points are formed. For eachspecific spacing of adjacent raster scan lines and number n of imagepoints, a second specific amount of a necessary displacement is derived,in parallel to the axis defined by the line of n image points, so that acontinuous imaging, i.e., the imaging of each raster point provided onprinting plate 28, is possible using the interleaving raster scan linemethod.

[0030] In one alternative exemplary embodiment, image points 210 canalso be moved in a meander shape over printing plate 28, in that acomplete imaging is initially carried out along one line in parallel toaxis of symmetry 25 of cylinder 26, and a step-by-step rotation issubsequently carried out about axis of symmetry 25 of cylinder 26.

[0031] It is clear that it is only a question of a relative movementbetween image points 210 and printing plate 28. This relative movementcan also be achieved by a movement of impression cylinder 26. For bothmoving directions of translation A and rotation B, it holds that themovements can take place continuously or step-by-step.

[0032] In another alternative specific embodiment, the device forimaging printing plates having light source 20, imaging optics 24 andthe like, can also be provided inside of impression cylinder 26, therebyachieving a space-saving configuration.

[0033] Prior to describing the interleaving raster scan line method indetail on the basis of a figure, general explanations in this regard areprovided. As already mentioned, to image a printing plate, the imagepoints are shifted over the printing surface, initially with a componentperpendicular to the direction defined by the line of the image points,in order to form so-called raster scan lines. A contiguous line of dotsis understood to be a line formed by the subsequent displacement in thedirection defined by the direction of the dots. In other words, the dotsare situated at the same level and belong to different scan lines thatare scanned next to one another.

[0034] The distances between the n image points simultaneously producedby the individual n laser diodes are selected to be constant; the lengthbetween two adjacent image points l is advantageously an integralmultiple m of pitch distance p between the dots, in other words l=m×p. Acontinuous inscription, i.e., each raster point is exposed at least onceto the image point of a laser, with n simultaneously scanned imagepoints at the distance l=m×p, m being a natural number and p thedistance between the dots, is always possible if one selects anappropriate displacement. The width of the displacement isadvantageously equal to the number of image points.

[0035] It can also happen, in this context, that one point is inscribedseveral times. A continuous inscription, in other words each dot isexposed exactly once, is especially possible when the number of imagepoints n and their spatial interval l, measured in units of the pitchdistance p of the dots, do not have a common divisor. Expresseddifferently, n and m are prime. This is the case, for example, when mand n are different prime numbers. At the same time, the displacement,which is stipulated by the direction given by the line defined by the nimage points, is to be selected as n. In the process, an edge area ofthe size r: r=n×m−(n+m−1) results at the beginning and end of the lineto be scanned.

[0036] Since each of the laser diodes can be driven discretely, each dotcan be configured individually. The performance of one specific laserbeam provided for inscribing a raster point is stipulated in accordancewith the image data information given. This enables the optical densityof different dots to be achieved on an individual basis.

[0037]FIG. 3 illustrates the interleaving raster scan line method forinscribing printing plates, based on an example of five image points,which are produced at the same time by the simultaneous irradiationusing five individual laser diodes. In this figure, dots are depicted insimplified form as small boxes. As already mentioned, each dot must beexposed once to an image point of a laser, so that it is exposed inaccordance with the image data given, or it can be left unchanged. Inthis example, a contiguous line to be scanned is composed of dotsdisposed side-by-side and without gaps in a row. Their pitch distance isdesignated by p. In FIG. 3, the group of simultaneously exposed dots 30is composed of five image points exhibiting a uniform spatial intervall. In first imaging 32, five unit points are exposed with spatialinterval l=3p. The group of simultaneously produced dots 30 is thendisplaced by five unit points, since, in this example, five dots aresimultaneously written in the direction defined by the axis of the dots;in this example, to the right. In second imaging step 34, five imagepoints are again set. A renewed displacement by five unit points followsiteratively in the direction defined by the axis of the dots; in thisexample, to the right. In imaging step 36 that follows, five points areset once again. From this sequence, it is apparent that the printingplate can be inscribed without gaps: each dot represented by a small boxis exposed once to the image point of a laser. In each renewed imagingfollowing a displacement step by five units of length, measured in unitsof p to the right, the same pattern is always produced at alreadyexposed and still unexposed dots, as is apparent in 38. In other words,at its right end, the line of exposed image points still has certaingaps of unexposed raster points. If, at this point, a further imaging offive raster points takes place at the right end, then the same sequenceof still unexposed and already exposed raster points is obtained. At thesame time, the portion of the line composed of completely inscribed dotsbecomes longer and longer. Likewise apparent in 38 is the edge area ofsize r, in this case eight dots, measured in units of pitch distance pof the dots.

[0038] Even in the event that individual single stripe laser diodes inthe array fail, the proposed interleaving raster scan line method can beused for scanning. Particularly when the number of n image points of thelaser beams and the spatial interval between two adjacent image pointsl, measured in units of p, are prime, the imaging speed is at a maximum.In other words, it is possible to specify an increment, so that eachpoint to be scanned is exposed only once to an image point of the laserbeams.

[0039] In the event that one or more of the single stripe laser diodesin the group of simultaneously scanned image points 30 is notfunctioning properly, it is still possible to inscribe using theinterleaving raster scan line method. In such an instance, it is alwaysthe largest section of the group having equidistant, adjacent imagepoints that is used for inscription. In order to achieve a continuousinscription, one must then obviously reduce the increment. It isbeneficial to do so in accordance with the above established rules withrespect to the properties of natural numbers.

[0040] The interleaving raster scan line method can be used to image aprinting plate for every combination of distances between adjacent imagepoints l and their number n. To continuously inscribe the printingplate, however, one must select appropriate parameters. If one imagepoint should drop out, an imaging at a reduced speed is possible.

[0041] The described interleaving raster scan line method requires amultiplicity of laser beams to inscribe a printing plate. These can alsobe produced by laser light sources other than the advantageously usedlaser diodes. To modify the projected distance between the individuallight sources, one advantageous refinement provides for tilting theprinting plate by an angle that differs from zero with respect to theplane disposed perpendicularly to the n laser beams.

[0042] Another advantageous refinement of the present invention providesfor a two-dimensional array of n₁×n₂ image points. When extrapolatingaccordingly, from one to two dimensions, spatial intervals l₁ and l₂between adjacent points in the two mutually perpendicular directionsmust be constant in each case, so that n₂ lines can be processed inparallel at distance l₂, in accordance with the discussedone-dimensional interleaving raster scan line method employing n₁ imagepoints at distance l₁. A displacement is then likewise carried out inthe perpendicular direction in accordance with the rules established forthe interleaving raster scan line method, in order to densely placedots.

[0043] Printing plates as defined wherein can include all types ofprinting forms.

Reference Symbol List

[0044]10 light source, individually drivable laser diode array

[0045]12 single stripe laser diodes

[0046]14 light beam

[0047]16 imaging optics

[0048]18 printing plate

[0049]110 image point

[0050]112 cooling element

[0051]114 current-supply and control line

[0052]116 control unit

[0053]118 temperature-control line

[0054]120 temperature control

[0055]122 detector for testing functioning and measuring power

[0056]124 connection to control

[0057]20 light source

[0058]22 laser beams

[0059]24 imaging optics

[0060]25 axis of symmetry

[0061]26 cylinder

[0062]28 printing plate

[0063]210 image points

[0064]212 path of image points

[0065] A translation

[0066] B rotation

[0067]30 group of simultaneously scanned dots

[0068]32 first imaging

[0069]34 second imaging

[0070]36 third imaging

[0071]38 iterative imaging

[0072] l spatial interval of the image points

[0073] p pitch distance of the dots

[0074] n number of image points

[0075] r edge area

What is claimed is:
 1. A device for imaging printing plates comprising:an array of n laser diodes which image n image points, so that one laserdiode of the array is allocated to each i-th point, with i being from{1, . . . , n}, the n image points being separated by a spatial intervall between adjacent image points, with a pitch distance p of dots to beimaged by the array, the laser diodes being individually-drivable singlestripe laser diodes.
 2. The device as recited in claim 1 wherein thespatial interval l between adjacent image points, measured in units ofthe pitch distance p of the dots, is an integral multiple m of the pitchdistance p between the dots.
 3. The device as recited in claim 2 whereinthe integral multiple m and the number n of image points have no commondenominator.
 4. The device as recited in claim 1 wherein the spatialinterval l of adjacent image points, measured in units of the pitchdistance p of the dots, is smaller than the number n of the imagepoints.
 5. The device as recited in claim 1 wherein the multiple m andthe number n of the image points are prime numbers.
 6. The device asrecited in claim 1 further comprising imaging optics for correcting atleast one of divergence and aberration.
 7. The device as recited inclaim 1 further comprising a control unit, at least one of the laserdiodes of the array being controlled by the control unit.
 8. The deviceas recited in claim 1 wherein the number of laser diodes in the array isbetween 10 and
 100. 9. The device as recited in claim 1 wherein thelaser diodes are spaced apart on the array by a distance of between 100and 1000 micrometers, and a width of emitter surfaces of the laserdiodes is less than 10 micrometers.
 10. The device as recited in claim 9wherein the width is 5 micrometers.
 11. The device as recited in claim 1further comprising at least one detector for testing for correctfunctioning and determining a power output of one or of a plurality ofthe laser diodes.
 12. The device as recited in claim 11 furthercomprising a laser controller, the laser controller being controlled asa function of the power output determined by the detector.
 13. Thedevice as recited in claim 1 wherein at least one laser diode is a pulsecontrolled laser.
 14. The device as recited in claim 13 wherein arepetition rate of the light pulses is at least exactly as great as apulse frequency of the pulse-controlled laser in order to displace theindividual dots.
 15. The device as recited in claim 1 further comprisingimaging optics including at least one reflective optical element. 16.The device as recited in claim 1 further including imaging optics havingmicro-optical components.
 17. The device as recited in claim 1 whereinthe printing plate is erasable or rewritable.
 18. An interleaving rasterscan line method for imaging printing plates by generating raster pointsusing an array of n laser light sources, which use an imaging optics toimage n image points arranged on a line, the n image points beingseparated by a spatial interval of adjacent points l, comprising thesteps of: simultaneously generating n image points on a printing plateby a plurality of laser light sources; generating a relative motionbetween the image points and printing plate; displacing the image pointswith a translation component perpendicular to an axis defined by theline of the image points by a first specific amount; displacing the nimage points in a direction defined by the n image points by a secondspecific amount; repeating the displacement steps, an amount of thesecond specific displacement being greater than the spatial interval lof adjacent image points.
 19. The interleaving raster scan line methodas recited in claim 18 wherein the second specific amount, measured inunits of the pitch distance p of dots to be imaged, is equal to thenumber n of image points.
 20. The interleaving raster scan line methodas recited in claim 19 wherein the spatial interval l of the imagepoints is an integral multiple of the pitch distance p of dots of thelaser diodes.
 21. The interleaving raster scan line method as recited inclaim 18 wherein the spatial interval l of the image points, measured inunits of a pitch distance p of dots of the laser diodes, and the numberof laser diodes n have no common denominator.
 22. The interleavingraster scan line method as recited in claim 21 wherein the spatialinterval l of the image points, measured in units of the pitch distancep of the dots, and the number of laser diodes are prime numbers.
 23. Aprint unit comprising at least one device for imaging printing plates,the device including an array of n laser diodes which image n imagepoints, so that one laser diode of the array is allocated to each i-thpoint, with i being from {1, . . . , n}, the n image points beingseparated by a spatial interval l between adjacent image points, with apitch distance p of dots to be imaged by the array, the laser diodesbeing individually-drivable single stripe laser diodes.
 24. A printingpress comprising at least one print unit in accordance with claim 23.